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Beginner

Scientific Name: Tetranychus urticae (Order: Acari; Family: Tetranychidae)

Identification

• Very small, with eight legs and no wings.
• Females measure up to 0.5 mm and males up to 0.3 mm.
• Females are oval in shape; males have a narrow abdomen and appear more active.
• Adults are pale yellow or yellowish-green with two dark spots on each side of their back. Overwintering females are orange or red in colour.
• Eggs are round, clear to pale-green and about 0.1 mm in diameter.
• Damage appears as white stippling or flecking on the upper leaf surface
• Damaged leaves will bronze and dry, die and fall off the plant, usually lower leaves first .
• Webbing is present across the lower surface of damaged leaves.

Often Confused With
Nutrient Deficiency
Drought Stress

Period of Activity:
Female TSSM overwinter in crop debris on the ground. In spring they move to new growth and feed on leaves closest to the ground. There are many generations per year and eggs, nymphs, and adults are frequently present at the same time. As populations build, leaves age or become damaged, mites move up the plant to newer, more succulent growth. The time for one generation to develop, from egg to mature adult, ranges from to 5 days at 24ºC to 3 weeks at 12ºC.

Scouting Notes
Raspberries grown in greenhouses or high tunnels can have major problems with mites, however most field-grown raspberry plantings will not have problems. Certain varieties like Autumn Britten are susceptible to mites in protected culture and in the field.

Use a 10-14X magnifying lens to check the lower leaf surface for mites and eggs. One of the first signs of infestation is the presence of webbing on the underside of leaves. Observe plants weekly for signs of mite damage, which first appears as flecks and stipples on the leaves. Collect leaf samples from mid-way along the cane and examine the lower surface for all stages of mites. Record the presence of beneficial insects such as syrphid larvae, ladybird beetle larvae and predatory mites.

Thresholds
There are no established thresholds for spider mite control on raspberries in Ontario. But if scouting reveals an average mite infestation of 10 to 15 mites per leaf, with no beneficials present, a miticide may be required.

 

Advanced

Scientific Name: Tetranychus urticae (Order: Acari; Family: Tetranychidae)

Identification
Two-spotted spider mites (TSSM) are tiny and spider-like, pale-green to straw-coloured with two dark spots on the back. TSSM are found mainly on plant foliage. The eggs are round, clear to pale-green and about 0.1 mm in diameter. After hatching, TSSM progress through three immature stages including a 6-legged larva, and an 8-legged deutonymph. Adult female mites, up to 0.5 mm in length, are oval in shape,  in contrast to the smaller and more active males, around  0.3 mm,  with a narrower  abdomen. Overwintering female mites are orange.  

Both adult mites and nymphs damage raspberry plants by sucking nutrients and chlorophyll out of leaves using their piercing-sucking mouthparts. They primarily feed from the underside of leaves, which causes white stippling or flecking on the upper leaf surface. Webbing is present across the lower surface of damaged leaves. Mite populations begin to build up on lower leaves, moving up to newer, more succulent growth as damage progresses.  Severely damaged leaves will begin to bronze and dry, die and fall off the plant. Spider mites can reduce fruit quality, yield, and overall plant vigour. Primocane growth may be stunted, which will adversely affect next year’s crop.

The infestation often begins at the edge of the field and in the lower canopy. Infestations are most severe in hot, dry, dusty conditions at the end of summer and exacerbated by low soil moisture. Severe damage from TSSM can predispose plants to winter injury.

Often Confused With

Nutrient Deficiency
Nutrient deficiency also leads to stunted growth, leaf chlorosis and necrosis. An infestation with two-spotted spider mites, however, is evident by the presence of mites, cast skins, and webbing on the underside of leaves.

Drought Stress
Plants suffering from drought stress also exhibit stunted growth and dried up leaves. Check the underside of leaves for webbing or the presence of mites to determine if the problem is a two-spotted spider mite infestation or drought stress.

Biology
Mature female mites overwinter in cracks of old fruiting canes and support posts, fallen leaves or protected ground cover.
Reproduction is continuous from spring to late fall, resulting in 5-15 generations per year, depending on temperature.

Colonization of raspberry plants begins in early spring as young leaves start emerging. Overwintering females place between 70 and 200 eggs on lower leaves in late April and May. Depending on the ambient temperature, hatching can take anywhere from 3 to 10 days. The life cycle takes between 2 to 3 weeks to complete at cooler temperatures (ie 12°C) but can be accomplished within as little as 5 days at temperatures of  25 to 35°C.

Female mites may live up to 30 days, however once temperatures drop and daylength decreases in the fall, the mites enter diapause, which is a period of rest between phases of growth and/or reproduction. The females change to a dark orange colour in fall and then hibernate.

Period of Activity:
Female TSSM overwinter in crop debris on the ground. In spring they move to new growth and feed on leaves closest to the ground. There are many generations per year and eggs, nymphs, and adults can be present throughout the growing season.  As populations build and leaves age or become damaged, mites move up the plant to newer, more succulent growth.

Scouting Notes
Most field-grown raspberry plantings will not have a problem with mites. However, when raspberries are grown in greenhouses or high tunnels, mites can be a major problem. Certain varieties like Autumn Britten are susceptible to mites both in protected culture and in the field. Mites outbreaks can also be triggered by the repeated use of insecticides which are harmful to beneficial insects.

Walk through the field and observe plants weekly for early signs of stippling, webbing  and bronzing on lower or middle aged leaves.  Collect leaf samples from mid-way along the cane and examine the lower surface for all stages of mites with a 10-14X magnifying lens. Record the presence of beneficial insects such as syrphid larvae, ladybird beetle larvae and predatory mites.

Thresholds
There are no established thresholds for spider mite control on raspberries in Ontario. But if scouting reveals an average mite infestation of 10 to 15 mites per leaf, and no beneficials, a miticide may be required.

Management Notes
Mite outbreaks in orchards can be caused by:

• Hot dry weather conditions that are favourable to mite development
• Poor groundcover management (providing overwintering sites for two-spotted spider mites)
• High nitrogen levels in leaves (resulting in higher mite fecundity)
• Loss of shelter and habit used by predators
• Use of pesticides toxic to natural enemies

Control mites if damage is evident and populations increase from week to week.

Beneficial insects reduce the need for miticides. Use pesticides which are not harmful to natural enemies such as ladybird beetles, lacewings, predatory mites and syrphid flies. Predatory mites can be purchased and released to help supplement naturally occurring predators. Predators should be applied at the first signs of TSSM activity.

There are several miticides available for raspberries. Each has a different mode of action and acts best against a certain stage of mite. Apply each product when most mites are at a susceptible stage for that product. Use different products from year to year to prevent the development of resistance.

Ensure adequate soil moisture and irrigate if necessary to prevent drought stress.

Reduce road dust.

Control and eliminate broadleaf weeds growing near raspberries as these can serve as alternate hosts to TSSM.

Do not apply excessive nitrogen.

If raspberries are grown in tunnels or a greenhouse, ensure that they receive adequate light and are well watered. New plants should be inspected for mites and infested plants should be bagged prior to their removal to prevent an accidental spread of mites.